Exhaust gas treating method

ABSTRACT

The present invention provides an exhaust gas treating method by which exhaust gas is passed through a catalyst layer at an areal velocity of 100 to 5000 m 3  /m 2  ·hr to selectively produce carbon monoxide from particulates contained in the exhaust gas, so that nitrogen oxides contained in the exhaust gas are removed by the carbon monoxide thus produced. The exhaust gas treating method of the present invention can readily and securely remove particulates and nitrogen oxides contained in exhaust gas, i.e., two primary causes of air pollution, by merely passing the exhaust gas through a catalyst layer at a specific areal velocity.

DESCRIPTION

1. Technical Field

The present invention relates to an exhaust gas treating method ofremoving particulates and nitrogen oxides from exhaust gas dischargedfrom a combustion engine such as a Diesel engine or the like.

2. Background Art

Exhaust gas discharged from a Diesel engine includes gas components suchas nitrogen oxides (NOx), sulfur oxides, carbon monoxide and the like,and fine powder having a grain size of not greater than 1 μm calledparticulates composed of soot (incomplete burnt carbon), soft or heavyhydrocarbon, sulfuric acid mist and the like.

The particulates are produced from the incomplete combustion of lightoil serving as fuel of a Diesel engine or the like. The nitrogen oxidesand the particulates are regarded as two primary causes of airpollution. It is therefore required to prevent the particulates and thenitrogen oxides from diffusing in the atmosphere.

In this connection, there are proposed various examples of a catalystdevice of which catalyst filter carrying an oxidation catalyst isinstalled at the exhaust system of a Diesel engine so that theparticulates from the Diesel engine come in contact with the catalystfilter, causing the particulates to be oxidized, decomposed and removed(See Japanese Unexamined Patent Publication 185425/1988, for example).

As a typical example of the catalyst device above-mentioned, there isknown, as shown in FIG. 1, a catalyst filter F in which an oxidationcatalyst is carried on a honeycomb structure 1 generally made of porousceramics which has openings at the exhaust gas inlet side A and openingsat the exhaust gas outlet side B, these openings at both sides A and Bbeing alternately closed.

In the catalyst filter F, exhaust gas introduced from the inlet side Ais forcibly passed through partition walls 2 made of porous ceramics anddischarged outside from the outlet side B, as shown by arrows. When theexhaust gas is passed through the partition walls 2, particulatescontained in the exhaust gas are caught by the partition walls 2 so thatthe particulates are oxidized and decomposed by the oxidation catalystcarried on the partition walls 2.

As the other primary cause of air pollution, nitrogen oxides which areto be removed by a reduction reaction, cannot be removed by theoxidation catalyst of the catalyst filter above-mentioned or the like.Accordingly, to remove both nitrogen oxides and particulates, both anoxidation catalyst and a reduction catalyst are required. This not onlycomplicates the exhaust gas treating mechanism, but also involves thelikelihood that the evacuation efficiency in a Diesel engine or the likeis lowered.

In view of the foregoing, the present invention is proposed with theobject of providing an exhaust gas treating method of readily andsecurely removing both nitrogen oxides and particulates contained inexhaust gas.

DISCLOSURE OF INVENTION

To solve the problems above-mentioned, the exhaust gas treating methodin accordance with the present invention is characterized in thatexhaust gas is passed through a catalyst layer at an areal velocity of100 to 5000 m³ /m². hr to selectively monoxide from particulatescontained in the exhaust gas, so that the particulates are removed andnitrogen oxides contained in the exhaust gas are removed by the carbonmonoxide thus produced.

According to the present invention having the arrangementabove-mentioned, when exhaust gas is passed through the catalyst layerat the areal velocity above-mentioned, incompletely burnt carbon orhydrocarbon in particulates contained in the exhaust gas is partiallyoxidized to selectively produce carbon monoxide, causing theparticulates to be removed. Further, the carbon monoxide thus producedcauses nitrogen oxides in the exhaust gas to be reduced and removed asdecomposed into nitrogen and water.

In the specification, the term of areal velocity refers to a valueobtained by dividing a space velocity, i.e., the flow velocity ofexhaust gas passing through the catalyst (in unit of hr⁻¹), by thecontact surface area per unit volume of the catalyst (in unit of m²/m³).

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a section view of a catalyst filter formed by a honeycombstructure;

FIG. 2 is a section view of a catalyst device incorporating the catalystfilter in FIG. 1; and

FIG. 3 is a section view of a catalyst device incorporating catalystfilters made of a wire net having a number of fine holes.

BEST MODE FOR CARRYING OUT THE INVENTION

No particular restrictions are imposed to the form of the catalyst usedin the practice of the present invention as far as exhaust gas can passtherethrough. Accordingly, there may be used conventional variouscatalyst devices arranged such that particulates come in contacttherewith, causing the same to be removed as oxidized.

For example, there may be used a catalyst device as shown in FIG. 2formed by mounting, in a case 3 having a predetermined shape, a catalystfilter F having the arrangement shown in FIG. 1 in which exhaust gas isadapted to be forcibly passed through partition walls 2 made of porousceramics so that particulates are oxidized and decomposed. Also, theremay be used a catalyst device as shown in FIG. 3 formed by mounting, ina case 3 having a predetermined shape, a plurality of catalyst filters feach made of a wire net or metallic sheet having, in the thicknessdirection, a plurality of fine holes with a diameter of not less than 30μm, the wire net or metallic sheet carrying a catalyst.

In the catalyst device in the form shown in FIG. 1, if exhaust gas comesin contact with the catalyst for a prolonged period of time, sulfuroxides contained in exhaust gas are oxidized by the catalyst so that thesulfur oxides are transformed into either sulfate which brings aboutacid rain, or sulfuric acid mist which becomes secondary particulates.To prevent such oxidation, it is preferred that the catalyst is carriedon only the surface layer portions of the partition walls 2 (of whichthe thickness from the wall surfaces is in a range from about 5 to about100 μm) by which the particulates are to be caught.

The areal velocity of exhaust gas passing through the catalyst layersshould be limited to a range from 100 to 5000 m³ /m² hr. Morespecifically, if the areal velocity is less than 100 m³ /m². hr, theperiod of time during which exhaust gas comes in contact with thecatalyst layers is too long. Accordingly, the oxidation of incompletelyburnt carbon or hydrocarbon contained in particulates is not limited topartial oxidation, but proceeds to such an extent as to produce carbondioxide. Therefore, the amount of the carbon monoxide is insufficient,failing to fully remove nitrogen oxides in the exhaust gas. Further, theoxidation of sulfur oxides contained in the exhaust gas proceeds toproduce sulfuric acid mist which becomes secondary particulates. Thisrather increases the ratio of particulates in the exhaust gas. On thecontrary, if the aereal velocity exceeds 5000 m³ /m². hr, the period oftime during which exhaust gas comes in contact with the catalyst layersis too short. This prevents the particulates from being sufficientlyoxidized, failing to sufficiently remove both the particulates and thenitrogen oxides.

Examples of the catalyst to be used in the present invention include:noble metal such as ruthenium, platinum, palladium, rhodium and thelike; oxides and composite oxides of base metal such as iron, chromium,copper, cobalt, manganese, nickel and the like; perovskite oxides suchas LaCoO₃ and the like.

INDUSTRIAL APPLICABILITY

According to the exhaust gas treating method of the present invention,particulates and nitrogen oxides contained in exhaust gas, i.e., twoprimary causes of air pollution, can be readily and securely removed bymerely passing the exhaust gas through a catalyst layer at a specificareal velocity. This eliminates such problems encountered inconventional methods as the complication of an exhaust gas treatingmechanism jointly using an oxidation catalyst and a reduction catalyst,the deterioration of evacuation efficiency in a Diesel engine or thelike, Further, the method of the present invention greatly contributesto the prevention of air pollution.

EXAMPLES

The following description will discuss the present invention withreference to Examples thereof and Comparative Examples.

EXAMPLE 1

10 Kg of activated alumina (A-11 manufactured by Sumitomo Chemical Co.,Ltd.), 1 kg of kibushi clay and 500 g of methyl cellulose were mixed ina dry state, and water was then added to the mixture. The resultantmixture was sufficiently kneaded. With the use of an extruder of theauger screw type having a honeycomb die having a pitch of 1.3 mm and awall thickness of 0.3 mm, the kneaded body was extruded and molded inthe form of a honeycomb. After dried with a drier of the ventilationtype, the resultant honeycomb body was put in an electric kiln, of whichtemperature was raised to 800° C. at a temperature raising ratio of 5°C. per hour. The honeycomb body was burning for one hour at 800° C. toprepare a cylindrical honeycomb structure having a number of honeycombholes and a diameter of 190 mm.

Slurry was prepared by adding 1000 g of activated alumina (A-11manufactured by Sumitomo Chemical Co., Ltd.) to 1 l of water in which 5g of ruthenium chloride had been dissolved. The slurry was dried andpulverized with a spray drier and then calcined in a nitrogen stream forone hour at 500° C. to prepare calcined powder. Together with 10 g ofsilica sol (Snowtex-O manufactured by Nissan Chemical Industries, Ltd.),100 g of the calcined powder was added to 500 g of water to prepare acatalyst-layer coating solution in the form of slurry.

The honeycomb structure was immersed in the catalyst-layer coatingsolution to apply the coating solution onto the surfaces of thehoneycomb structure. Air was blown to the honeycomb structure to removethe excessive application of the coating solution. The honeycombstructure was dried with a drier of the ventilation type to prepare analumina-ruthenium carrying honeycomb structure. The alumina-rutheniumlayers serving as catalyst layers carried on the honeycomb structure hada thickness of about 20 μm. According to a geometrical calculation, thespecific contact area of each alumina-ruthenium layer was 1190 m² /m³.

The cylindrical alumina-ruthenium carrying honeycomb structure was cutinto round slices each having a thickness of 17 mm. There was prepared acatalyst filter F, as shown in FIG. 1, of the type in which exhaust gasis adapted to be forcibly passed through partition walls 2, with halfthe number of entire honeycomb holes closed at one side of the filter Fwith alumina cement and the other half the number of entire honeycombholes closed at the other side of the filter F with alumina cement.

As shown in FIG. 2, the catalyst filter F thus prepared was mounted in acase 3 to make a catalyst device.

EXAMPLE 2

There was prepared a catalyst filter, which was then mounted in a caseto make a catalyst device in the same manner as in Example 1, exceptthat the cylindrical alumina-ruthenium carrying honeycomb structureabove-mentioned was cut into round slices each having a thickness of 35mm.

EXAMPLE 3

There was prepared a catalyst filter, which was then mounted in a caseto make a catalyst device in the same manner as in Example 1, exceptthat the cylindrical alumina-ruthenium carrying honeycomb structureabove-mentioned was cut into round slices each having a thickness of 105mm.

COMPARATIVE EXAMPLE 1

There was prepared a catalyst filter, which was then mounted in a caseto make a catalyst device in the same manner as in Example 1, exceptthat the cylindrical alumina-ruthenium carrying honeycomb structureabove-mentioned was cut into round slices each having a thickness of 300mm.

EXAMPLES 4 TO 9

Slurry was prepared by adding 1000 g of activated alumina (A-11manufactured by Sumitomo Chemical Co., Ltd.) to 1 l of water in which 5g of platinic chloride had been dissolved. The slurry was dried andpulverized with a spray drier and then calcined in a nitrogen stream forone hour at 500° C. to prepare calcined powder. Together with 10 g ofsilica sol (Snowtex-O manufactured by Nissan Chemical Industries, Ltd.),100 g of the calcined powder was added to 500 g of water to prepare acatalyst-layer coating solution in the form of slurry.

Cylindrical honeycomb structures, each of which was identical with thatused in Example 1 having a diameter of 190 mm and a number of honeycombholes, were subjected to immersion in the catalyst-layer coatingsolution above-mentioned, air blowing to remove the excessiveapplication of the coating solution and drying with a drier of theventilation type. The honeycomb structures were then calcined in anitrogen stream for one hour at 500° C. and then in a nitrogen-hydrogen(1:1) stream for one hour at 500° C., thereby to preparealumina-platinum carrying honeycomb structures having, as catalystlayers, alumina-platinum layers respectively having the thicknessesshown in Table 1.

Each of the cylindrical alumina-platinum carrying honeycomb structurethus prepared was cut into round slices each having a thickness of 17mm. In the same manner as in Example 1, there were prepared catalystfilters, which were then respectively mounted in cases to make catalystdevices.

                  TABLE 1    ______________________________________            Thickness of Alumina-Platinum Layer    ______________________________________    Example 4 10 μm    Example 5 25 μm    Example 6 50 μm    Example 7 75 μm    Example 8 100 μm    Example 9 200 μm    ______________________________________

EXAMPLE 10

A wire net made of stainless steel (SUS 304) having a wire diameter of260 μm and meshes of 420 μm was cut into a disk having a diameter of 190mm. The disk-like wire net was degreased as heated for 30 minutes at400° C. The wire net was then immersed in a catalyst-layer coatingsolution identical with that used in Examples 4 to 9 to prepare acatalyst filter made of an alumina-ruthenium carrying wire net. Theratio of the catalyst carried on the wire net was 9.8%.

As shown in FIG. 3, ten catalyst filters f each of which was made in themanner above-mentioned were mounted, at regular spatial intervals of 5mm, in a case 3 to make a catalyst device.

COMPARATIVE EXAMPLE 2

A catalyst device was made in the same manner as in Example 10, exceptthat two catalyst filters made of the alumina-ruthenium carrying wirenet prepared in Example 10, were mounted in a case at regular spatialintervals of 5 mm.

EXAMPLE 11

A wire net made of stainless steel (SUS 304) having a wire diameter of260 μm and meshes of 420 μm was cut into a disk having a diameter of 190mm. The disk-like wire net was degreased as heated for 30 minutes at400° C. The wire net was spray-coated at the surfaces thereof with 50 gof alumina, and then immersed in a mixed aqueous solution containingcopper nitrate, chromium nitrate and manganese nitrate at a ratio byweight of 2.7: 2.0: 95.3 in terms of oxides, thereby to prepare acatalyst filter made of an alumina-metallic oxide carrying wire net. Theratio of the total carried metallic oxides (three components) withrespect to carried alumina was 5% by weight.

As shown in FIG. 3, ten catalyst filters f each of which was made in themanner above-mentioned were mounted, at regular spatial intervals of 5mm, in a case 3 to make a catalyst device.

EXAMPLE 12

A mixed aqueous solution containing lanthanum acetate, cobalt acetateand zirconium hydroxide at a ratio by weight of 2.7: 2.0 : 95.3 in termsof oxides, was evaporated and solidified. The resultant solid body wascalcined in air for one hour at 850° C. to prepare a perovskite oxide(LaCoO₃ /ZrO₂). The perovskite oxide was pulverized with the use of asample mill.

Together with 10 g of silica sol (Snowtex-O manufactured by NissanChemical Industries, Ltd.), 100 g of the pulverized powder was added to500 g of water to prepare a catalyst-layer coating solution in the formof slurry. Then, there was made, in the same manner as in Example 10, acatalyst filter made of a LaCoO₃ /ZrO₂ carrying wire net. The ratio ofthe catalyst carried on the wire net was 8.5%.

As shown in FIG. 3, ten catalyst filters f each of which was prepared inthe manner above-mentioned, were mounted, at regular spatial intervalsof 5 mm, in a case 3 to make a catalyst device.

EVALUATION TEST

Each of the catalyst devices of Examples and Comparative Examples thusprepared was mounted on the downstream part of a Diesel engine (6HADKmanufactured by Yanmar Diesel Engine Co., Ltd.) presenting a strokevolume of 12 l and an exhaust gas amount of 700 Nm³ /hr.

While each catalyst device was heated from the outside to adjust thereaction temperature, the Diesel engine was operated at the number ofrevolutions of 2000 r.p.m. and under drive torque of 100 kg.m. Under theoperting conditions above-mentioned, the exhaust gas presented thefollowing composition:

Nitrogen oxides (NOx) 500 ppm

Sulfur oxides (SO₂) 150 ppm

Carbon monoxide 300 ppm

Oxygen 5%

Water 10%

The average concentration of particulates in the exhaust gas was 1.0g/Nm³.

While the Diesel engine was operated under the conditionsabove-mentioned, there were obtained the ratios of the particulates andthe nitrogen oxides removed by each catalyst device.

The particulate removal ratio was obtained as follows. With the use of aso-called dilution tunnel method by which the exhaust gas from a Dieselengine is diluted to generate a situation similar to that generated atthe time when the exhaust gas is actually discharged to the atmosphere,and the amount of particulates is measured in such a situation (See"Investigation of Documents relating to Technique of Lowering BlackSmoke from Diesel Engine" published by Nihon Kagaku Gijyutsu JyohoCenter in March 1984), there was measured the weight of fine powdercollected by the filter having a thickness of 47 μm mounted on theoutlet of each catalyst device. Based on the value thus obtained, theparticulate removal ratio was calculated.

The nitrogen oxide removal ratio was calculated from the measured resultof nitrogen oxides by an NOx analyzer.

The results are shown in Table 2.

                  TABLE 2    ______________________________________           Areal      Particulate                                Nitrogen Oxide           Velocity   Removal   Removal           (m.sup.3 /m.sup.2  · hr)                      Ratio (%) Ratio (%)    ______________________________________    Example 1             2964         30.1      19.9    Example 2             988          62.0      41.4    Example 3             282          55.2      23.8    Comparative              99          -10.3     3.8    Example 1    Example 4             988          42.8      25.0    Example 5             988          53.2      40.7    Example 6             988          57.5      56.9    Example 7             988          53.1      62.3    Example 8             988          32.6      54.2    Example 9             988          8.1       26.8    Example 10             3527         73.6      69.3    Comparative             17635        6.7       9.2    Example 2    Example 11             3527         59.8      44.0    Example 12             3527         60.7      40.9    ______________________________________

As apparent from Table 2, with Comparative Example 1 where the exhaustgas areal velocity was not greater than 100 m³ /m². hr, the particulateswere rather increased in amount and the nitrogen oxides were notsufficiently removed. With each of Examples 1 to 3 where there was usedthe catalyst filter made of the alumina-ruthenium carrying honeycombstructure identical with that used in Comparative Example 1 and wherethe exhaust gas areal velocity was in a range from 100 to 5000 m³ /m².hr, the particulates and the nitrogen oxides were sufficiently removed.From the results of Example 10 and Comparative Example 2, it was foundthat, if the exhaust gas areal velocity exceeded 5000 m³ /m². hr eventhough the catalyst filter made of the same alumina-ruthenium carryingwire net was used, the removal ratios of particulates and nitrogenoxides were considerably lowered. From the results of Examples 4 to 9,it was found that, if the thickness of the alumina-platinum catalystlayer exceeded 100 μm even though the catalyst filter made of the samealumina-platinum carrying honeycomb structure was used, the particulateremoval ratio was lowered. Thus, it was found that, when a honeycombstructure was used, it was preferable that the thickness of the catalystlayer was not greater than 100 μm.

From the entire results in Table 2, it is found that the exhaust gastreating method of the present invention may be applied to variouscatalyst devices in which various catalyst layers are carried.

We claim:
 1. In the method of treating an exhaust gas containingparticles of hydrocarbonaceous and carbonaceous matter, well as nitrogenoxides, by passing said gas through a catalyst comprising oxidationcatalyst means under conditions sufficient to entrap and oxidize saidcarbonaceous particles, the improvement, whereby removing both saidnitrogen oxides and particulates, which comprises passing said exhaustgas through said catalyst at an areal velocity of 100 to 5,000 m³ /hr²under a combinaton of conditions sufficient to partially oxidize saidhydrocarbonaceous and carbonaceous matter in said particles to anoxidation product comprising carbon monoxide and water, and reactingsaid nitrogen oxides with said carbon monoxide sufficient to reduce saidnitrogen oxides and to oxidize said carbon monoxide.
 2. An improvedmethod as claimed in claim 1 wherein said oxidation catalyst is at leastone member selected from the group consisting of a noble metal, a basemetal oxide, a composite metal oxide, and a perovskite crystal.
 3. Animproved method as claimed in claim 1 wherein said catalyst comprises analuminum-platinum layer carried on the surface portion of a honeycombstructure.
 4. An improved method as claimed in claim 3 wherein thethickness of said aluminum-platinum layer is not greater that 100 μm. 5.An improved method as claimed in claim 1 wherein said catalyst iscarried on a wire net.
 6. An improved method as claimed in claim 1wherein said catalyst is carried on a metal sheet perforated with fineholes.